1. Field of the Invention
The present invention relates to a head-rest device for use in a vehicle seat, and, more particularly, to an improved head-rest device which is capable of adjustment in height as required.
2. Description of the Prior Art
When using this type of head-rest, it is necessary to adjust the height of the head-rest in a vertical direction so as to suit various occupants with different sitting heights and this vertical adjustment must be performed manually. In this sense, it is preferred to carry out such adjusting operation using a light force. When shocks are applied to the head-rest device by the head of the occupant in case of a rear-end collision or the like, however, forces tending to downwardly push the head-rest device will be produced and these downward forces are often considerably great. Accordingly, if the head-rest device is so constructed as to be adjustable in height with a small pressure, then its height may be lowered unnecessarily when any shocks are applied to the head-rest and thus the head-rest device cannot maintain the safety of the occupants sufficiently. In other words, such head-rest device is found incapable of satisfying the dual requirements, i.e., the light-pressure operationability and the safety maintenance function simultaneously.
Therefore, there have been conventionally proposed various head-rest devices of a type which includes a lock mechanism provided between head-rest stays and head-rest holders so as to satisfy both of the above-mentioned two conflicting requirements in a synchronous manner, that is, which is easily adjustable in height as well as is capable of maintaining its predetermined position against any possible shocks applied from above. However, such conventional head-rest devices are complicated in structure and the lock mechanisms employed therein have been found rather difficult to operate. In addition, since such prior art head-rest devices include locking means so constructed as to lock the head-rest stays in a downward direction only, when large vibratory loads are applied to the associated seat, the head-rest stays are caused to slide upwardly so that the head-rest cannot be supported in a stable manner.